This invention relates to an airlift membrane device, an airlift membrane bioreactor containing same, and an airlift bioreactor process. The membrane device utilizes one or more multiple passageway porous monoliths as a microfiltration or ultrafiltration membrane support. The monolith-based membrane device provides a compact, low cost device that has well-controlled and efficient airlift for membrane flux maintenance. The use of a ceramic membrane offers a hydrophilic membrane resistant to fouling by the bioreactor biomass feed stock.
The rapid emergence of the membrane bioreactor (MBR) has lead to the deployment of several types of membrane devices in such MBR""s, in both xe2x80x9csubmerged membranexe2x80x9d and pumped xe2x80x9cexternal loopxe2x80x9d membrane module configurations. For the submerged membrane configuration, which is favored due to lower costs, there are primarily two membrane types employed: polymeric hollow fibers and polymeric plate devices. Descriptions of the state of the art for both submerged and external loop technology can be found in the following:
1. Articles in the June 2002 issue of Filtration+Separation, Vol. 39, no. 5, pages 26-35.
2. Proceedings of the Microfiltration III Conference, Costa Mesa, Calif., May 5-7, 2002.
3. xe2x80x9cMembrane Bioreactors: Wastewater Treatment Applications to Achieve High Quality Effluentxe2x80x9d, by Steven Till and Henry Mallia, presented at the 64th Annual Water Industry Engineers and Operatorsxe2x80x99 Conference, Sep. 5-6, 2001, Bendigo, Australia.
The last paper describes the two leading submerged systems, hollow fibers sold by Zenon (Canada) and plate devices sold by Kubota (Japan). The invention that is the subject of this patent application and that can be used in a submerged MBR is a substantially different membrane configuration, viz. a multiple passageway monolith membrane device. The structures covered by this invention have the characteristics of intrinsically low cost and a very high membrane surface area per unit volume of the device.
Similar devices in various structures when used as crossflow membrane modules, as could be used in external loop MBR""s, have been disclosed in the following patents, specifically incorporated herein by reference:
1. U.S. Pat. No. 4,781,831 (Goldsmith), which discloses in FIG. 5 therein, and described in the patent Specification, a cluster of individual multiple passageway monoliths arranged to have xe2x80x9cfiltrate flow conduitsxe2x80x9d formed by the space among the monolith elements.
2. U.S. Pat. No. 5,009,781 and U.S. Pat. No. 5,108,601 (Goldsmith), which therein disclose in the Figures and Specification unitary monolith structures with filtrate conduits formed within the monoliths.
3. U.S. Pat. No. 6,126,833 (Stobbe, et al.), which discloses structures comprised of a collection of monolith segments containing both segment internal filtrate conduits and a filtrate conduit arrangement formed by the gap among the monolith segments.
Preferred embodiments of the monolith based membrane device would be fabricated from a porous ceramic monolith support and a finer-pored ceramic or polymeric membrane coating applied to the passageway wall surfaces of the monolith support.
Ceramic membrane microfiltration (MF) and ultrafiltration (UF) devices have been used in external MBR systems. Examples are found in an article by Wen, Xing, and Qian (xe2x80x9cCeramic Ultra Filtration Membrane Bioreactor for Domestic Wastewater Treatmentxe2x80x9d, Tsinghau Science and Technology, ISSN 1007-0214, 08/17, Vol. 5, No. 3, pp 283-287 (September 2000)) and an article by Fan, Urbain, Qian, and Manem (xe2x80x9cUltrafiltration of Activated Sludge with Ceramic Membranes in a Cross-Flow Membrane Bioreactor Processxe2x80x9d, Water Science and Technology, Vol. 41, No. 10-11, pp 243-250 (2000)).
There has been little work using ceramic membranes in a submerged MBR configuration. A recent presentation by Xu, Xing, and Xu entitled xe2x80x9cDesign and Application of Airlift Membrane-Bioreactor for Municipal Wastewater Reclamationxe2x80x9d describes the use of an airlift MBR using single tubular ceramic UF membrane elements and a five (5) channel multichannel UF membrane element (Presentation at the North American Membrane Society Meeting, May 11-15, 2002, Long Beach, Calif.).
This device features a submerged, vertically-mounted airlift membrane device. The device comprises a structure of one or more monolith segments of porous material, each monolith segment defining a plurality of passageways extending longitudinally from a bottom feed end face to a top retentate end face. The surface area of the passageways in the monolith segment is at least 150 square meters per cubic meter of monolith segment volume, and the porous material has a porosity of at least 30% and a mean pore size of at least 3 xcexcm porous membrane with mean pore size below 1 xcexcm is applied to the walls of the monolith segment passageways to provide a separating barrier. A gas sparger is located below the device to provide a gas-sparged liquid feed stock at the bottom end face to provide airlift circulation of the feed stock through the device, which separates the feed stock into filtrate and a residual gas-containing retentate that passes from the top end face of the device. At least one filtrate conduit is formed within the device for carrying filtrate from within the device toward a filtrate collection zone of the device, the filtrate conduit providing a path of lower flow resistance than that of alternative flow paths through the porous material. The device has at least one seal to separate feed stock and retentate from the filtrate collection zone.
In a preferred embodiment, the porous material of the membrane device is ceramic. The device structure can be comprised of a single monolith or an assembly of monolith segments. The membrane device can be contained in a housing for filtrate collection and the filtrate collection zone is the annular space between the device and the housing. Alternatively, the device can be isolated along the exterior surface and the filtrate can be withdrawn from an end face of the device.
The membrane used in the device can be a microfiltration membrane with a pore size from about 0.1 to about 1 micron or an ultrafiltration membrane with a pore size from about 5 nm to about 0.1 micron. Preferably, the membrane is a ceramic membrane.
The vertically mounted membrane device can contain a shroud extending below the bottom end face of the device and the gas is sparged into a cavity created by the shroud. Preferably, the hydraulic diameter of the passageways is from about 4 to 15 mm and the preferred hydraulic diameter of the monolith segments is greater than about 50 mm.
This membrane device can be used in a membrane bioreactor that includes, in addition to the cross flow membrane device, a membrane bioreactor feed tank with means of introduction of a liquid feed stock and a means to convey the filtrate from the filtrate collection zone of the device to the filtrate discharge point of the bioreactor.
The membrane device can be installed within a bioreactor feed tank in an internal airlift circulation loop, or it can installed external to the feed tank in an external airlift circulation loop. The sparged gas can be air or oxygen and the bioreactor can operate under aerobic conditions, or the sparged gas can have low or negligible oxygen content and the bioreactor can operate under anaerobic conditions.
This invention further features a bioreactor process that includes introducing a feedstock into a submerged airlift membrane bioreactor. Gas is sparged at a bottom feed inlet of at least one submerged, vertically-mounted membrane device to provide airlift circulation of the feedstock through the device, and the feed stock is separated into filtrate and residual gas-containing retentate which passes from the top end of the device. The device consists of a structure of one or more monolith segments of porous material each monolith segment defining a plurality of passageways extending longitudinally from a bottom feed end face to a top retentate end face, the surface area of the passageways in the monolith segment being at least 150 square meters per cubic meter of monolith segment volume. The porous material has a porosity of at least 30% and a mean pore size of at least 3 xcexcm and a porous membrane with mean pore size below 1 xcexcm is applied to the walls of the monolith segment passageways to provide a separating barrier. At least one filtrate conduit within the device carries filtrate from within the device toward a filtrate collection zone of the device, and the filtrate conduit provides a path of lower flow resistance than that of alternative flow paths through the porous material. The device has a means to separate feed stock and retentate from the filtrate collection zone. The filtrate collected in the filtrate collection zone is conveyed to the filtrate discharge point of the bioreactor.